The physical conditions in the PDR of W49N

We present ISO-LWS and ground-based SEST observations which probe the Photo Dissociation Region (PDR) of W49N. The ISO-LWS observations consist of low resolution (similar to 200) grating spectra between 43 mum and 197 mum, an d high resolution (similar to 10(4)) Fabry-Perot spectra of the [CII] 158 m um and [OI] 63 and 145 mum lines. The SEST observations consist of (CO)-O-1 6 J = 1 -->0 and (CO)-O-18 J = 2 -->1 maps of the region. Using all these o bservations and with the help of a detailed PDR model, we have determined t he physical and chemical structure of the PDR of W49N. We find that only a tiny fraction, 2.5%, of the cloud gas is in the atomic form. This gas has a n average density n=10(4) cm(-3) and a temperature T similar to 130 K. The PDR is created by a FUV field G(0) = 3x10(5) times brighter than the averag e FUV interstellar field, which is emitted by the stars associated with the several HII regions inside the cloud. The photoelectric heating efficiency is very low, similar to 10(4), relative to other known PDRs, probably beca use of the relatively high G(0)=n ratio. Overall, W49N seems to be one of t he youngest giant molecular clouds actively forming stars in the Galaxy, si nce most of the cloud material is still molecular and dense and not yet dis persed by the several embedded newly formed massive protostars.